Export thread

I am putting this in the Briggs section but it could be under any brand. How do you folks generally deal with a bent shaft?

There are the 4 ways that I know of repairing it:

1. Replace the Crank shaft
2. Use a long pipe and try to bend it back to something usable.
3. Hit it with a Big hammer. Hope you do not break anything and make it usable
4. Use a fixture to bend it back. most people do not have access to such a tool.

Is there another way I am missing? In most cases I would bet most have walk away and junked the mower or the engine.

But I may be wrong?. share what you do....

Option 1 is not cost feasible in most cases. And the other 3 are a liability risk for most shops and won't happen

I doubt that you will find anyone here recommending any of the procedure you listed except junking. I have straightened them but *only* using a friends straightening fixture made for that express purpose. I made my own specifically for straightening Lawn Boy F series crankshafts which always seemed to be bent but I would not recommend that anyone else do either. You will likely hear of all kinds of terrible results from straightening, I never had first hand knowledge of any disaster in yea many years of experience.

Walt Conner

And here is tech number 3 saying the same thing
Once the crankshaft bends, its microstructure is compromised so it will forever be weak & highly prone to bending again
Straitening jigs rarely get a crank 100% true and a crank that is not true will shake like Elvis's hips if it is bad down to a light tingle in the bars if it is good .
Even a good one , unless it is true to within 0.0005 ", the bent crank will gouge out the bearing surface at either end so down the track a bit the engine will do a seal then seize from oil loss before you notice it .
DIY then replacement cranks are feasible remembering it will be new seals & gaskets as well
Taking it to a workshop the repair will be around 75% or more of the cost of fitting a new engine.

bertsmobile1 said:

Once the crankshaft bends, its microstructure is compromised so it will forever be weak & highly prone to bending again
Straitening jigs rarely get a crank 100% true and a crank that is not true will shake like Elvis's hips if it is bad down to a light tingle in the bars if it is good .
Even a good one , unless it is true to within 0.0005 ", the bent crank will gouge out the bearing surface at either end so down the track a bit the engine will do a seal then seize from oil loss before you notice it .

Click to expand...

It depends. If there are cracks it is a bad idea to bend it or use it. Bending it with out any visible cracks will work harden the steel and make it harder and less prone to bending in the future. At least in the same place.

I have bent 3/4 diameter shafts so that the vibration is not noticeable by the average end user. Not perfect but usable and back in service with no issues. Will it take some life out of the bearings? Very likely. But you are giving some life back to something that was dead. It is all bonus life. That is my practical point of view. Getting it back to .0005 TIR or close to it is not going to happen.

I know of two people who have bent their shafts and still use the mower as it is. It vibrates a bit more than normal but they live with it. Nothing bad has happened so far. One is a $600 Honda and the other is a cheap Poulan.

I know most larger shops or dealers will not do it. They want to sell you a new machine. They will take your junk mower off your hands.

I know of one independent shop in my area who will not do it for a customer, but he does it for himself. He will resell mowers that had bent shafts that he has straightend. How do I know this? He as much as told me.

Stens sells a fixture and the last time I looked at it the price was around $350.

I browsed the post. If the crank is bent replace it or junk the engine. The next post will be "it only vibrates when it is running... "

Personally I haven't seen but a handful of bent shafts in the 13+ yrs I have been servicing mowers for others. No way I would ever recover the cost of a straightener jig. Now of course fabricate my own jig but why as it would just take valuable space in my shop collecting dust after taking a week to it designed right.

I once tried refurbishing mowers and sell them with a 90 days warranty. All I got was insult offers so I will not waste my time or money doing it. I just part them out as I am a repair only shop now with a few parts only sells.

nbpt100 said:

It depends. If there are cracks it is a bad idea to bend it or use it. Bending it with out any visible cracks will work harden the steel and make it harder and less prone to bending in the future. At least in the same place.

I have bent 3/4 diameter shafts so that the vibration is not noticeable by the average end user. Not perfect but usable and back in service with no issues. Will it take some life out of the bearings? Very likely. But you are giving some life back to something that was dead. It is all bonus life. That is my practical point of view. Getting it back to .0005 TIR or close to it is not going to happen.

I know of two people who have bent their shafts and still use the mower as it is. It vibrates a bit more than normal but they live with it. Nothing bad has happened so far. One is a $600 Honda and the other is a cheap Poulan.

I know most larger shops or dealers will not do it. They want to sell you a new machine. They will take your junk mower off your hands.

I know of one independent shop in my area who will not do it for a customer, but he does it for himself. He will resell mowers that had bent shafts that he has straightend. How do I know this? He as much as told me.

Stens sells a fixture and the last time I looked at it the price was around $350.

Click to expand...

The question are you willing to risk several million dollar lawsuit if something goes wrong. And yes Stens sells a straightener. But take into account that most mowers that end up with bend cranks are less than $200 for the average customer. And take into account that using the straightener requires removing the engine and putting it into the jig. and the going through the straightening procedure which by this time has taking up to a couple hours. With in shop time could be $150-200 so you have exceeded the replacement cost of the mower. For the $600 mowers they would be more worth just replacing the crankshaft on a new mower, not one with several years of run time

And for the record I don't sell new or used mowers. Too many shops around that sell new stuff, which I will work on after the warranty. And like somebody else say used stuff just gets you lowballed on selling price. How many straightened crankshaft mowers are willing to sell for $15-20 each. Because that is the backyard selling price for people to put on the curb with a for sale sign in my area. Or pick them up at the local community auction for $5 a piece.

Well I will put my metallurgists hat on and tell you that you are so wrong it is not funny
There are 2 common crankshafts , forged steel or cast malleable iron
Both are used in mower engines
Forged steels do not work harden unless they are a molly grade as used in tractor blades and some mower blades .
Cast cranks will recrystalize where it bent originally
It was spinning when it bent so it bent in a high energy bi-axial load situation somewhat similar to spin forming
The straitening jigs are static load so they put a lot of stress into the crank
In either case the worked region will have a different crystal macro structure so it will no longer be uniform top to bottom or around the circumference or radially .
If you were to cut through the crank, polish then etch & look at the macrostructure the damage would be quite evident .
I have done this sort of stuff for near a decade to establish if what the driver of the vehicle that caused the collision was telling the truth ( usually not ) or the recient repairs had been done properly ( again usually not ).
Just because some one managed to force the crank back into reasonable shape and the mower did not immediatley disentergrate does not mean it will not happen in the future
Some times you can be lucky and some times you are not lucky but the consequences of the crank shaft breaking while you are walking behind it do not stack up comparred to the cost of replacing the crank.
Or as I often say to customers the price of the part is less than the taxi fare back from the ER let alone the ambulance to get you there in the first place .

The purpose of this forum is to give sound honest advice that is in the best interest of the poster and that is what they got from 5 techs who have nothing to gain from them replacing the crank or the mower so on the face of it should be good advice
IF nbpt want to try his luck he is more than welcome to do it but 5 techs have all advised against it ,

Now You tube is full of ego maniacs desperate to prove they have more than 2 functioning brain cells so it will be awash with "how to straiten a bent crank shaft " videos but you can be sure none will be posted by engineers or metallurgist who actually know & understand why it is not a good idea .

Gee Bert you give them a lot more credit. I was thinking they barely had one working brain cell. I know they get on my last working nerve here. If I could would depot to the Sun where at least they would of some use.

Usually with walk behind the replacement costs are just as low as the repair or less because the way they dump the mower on the market. Besides the customer get a warranty and it least has all new parts.

Just as a point of curiosity, they did make a straightener with which you did not remove the engine. I never used one but did see one at a local Western Auto Store repair shop many years ago.

i mentioned that I made a straightener just for LBs. This was years ago when LBs were in high demand for home owner use. I bought every used LB I could find including from a commercial user and, as I said, invariably the "F" series had bent crankshafts. The metering arm on commercial straighteners was about 6-8" long, mine was 2 ft. long. I got very good accuracy. Could not straighten "D" series, too hard and actually broke my straightener. I often said "F" series crankshafts were as hard as a wet noodle.
Walt Conner

I did not ask this question from the business model point of view. i.e. Can you (should you) make money doing it? No. I was curious how people repair it or get around the problem.

I know people do it for various reasons. Not always to make money. This can be a long and detailed topic so I will stop here.

I know straitening can work in some instances. By the same token it can be catastrophic and you need to use sound judgement. The financial services industry has hijacked our society so everyone is to think litigiously. People are always worried about some kind of legal liability. You better run out and buy more insurance. The lawyers are all over tv now fishing for business. At least in the USA. Who wins? The Financial Services industry and a bunch of lawyers. But this is an other story for another board.

Bert, thanks for your technical expertise and experience. However I will agree to disagree on one point. A cast crank will work harden (strain harden may be the term you are more familiar with). The nodular cast iron used on cranks is designed to be ductile and bend rather than break like most cast irons. Also it is cheaper that forged, so I will stick my neck out and guess all walk behind lawn mowers have cast crankshafts. Someone will correct me if I am wrong. Please don't chop off my head. Work hardening is not something you would intentionally do on this material, but it work hardens when strained. Just science.

Walt, I would love to see a picture of your fixture.

This forum is setup to help members repair their equipment. The vast majority of solutions are given by the members with a great amount of expertise in this field. When we see something which has the probability of causing harm we have to educate members that the information they requested is not something they would recommend anyone do. You are going to be given information based on business practices, because that is what we do. I doubt anyone of the experts who posted their opinions were thinking much about liability, but more able the safety of the questioner. Yes we all have liability insurance, but that is a requirement of doing this work, not because we worry about who might sue us. If we would worry about that, we would get out of the business. Our#1 goal is to provide good quality work we can stand behind and put food on the table for our families. Most DIY guys, who post here, are looking to fix things the cheapest way possible and never really think about the consequences. This is where we hope they learn from our non repair answers, as they are just as important as “you fix it this way”. Just my opinion after reading all the posts on this thread and deciding it was time to add my $.02.

I have the crankshaft straightening jig that works without removing the crank from the engine. I quit using it years ago. I straightened a bunch that were slightly bent and they worked fine. If bent more more than just a little the crank seal usually failed or the bottom case bearing failed. Not worth the effort or liability anymore. As a shop i won't do it.

nbpt100 said:

I did not ask this question from the business model point of view. i.e. Can you (should you) make money doing it? No. I was curious how people repair it or get around the problem.

I know people do it for various reasons. Not always to make money. This can be a long and detailed topic so I will stop here.

I know straitening can work in some instances. By the same token it can be catastrophic and you need to use sound judgement. The financial services industry has hijacked our society so everyone is to think litigiously. People are always worried about some kind of legal liability. You better run out and buy more insurance. The lawyers are all over tv now fishing for business. At least in the USA. Who wins? The Financial Services industry and a bunch of lawyers. But this is an other story for another board.

Bert, thanks for your technical expertise and experience. However I will agree to disagree on one point. A cast crank will work harden (strain harden may be the term you are more familiar with). The nodular cast iron used on cranks is designed to be ductile and bend rather than break like most cast irons. Also it is cheaper that forged, so I will stick my neck out and guess all walk behind lawn mowers have cast crankshafts. Someone will correct me if I am wrong. Please don't chop off my head. Work hardening is not something you would intentionally do on this material, but it work hardens when strained. Just science.

Walt, I would love to see a picture of your fixture.

Click to expand...

Nodular iron, malleable iron ductile SG iron basically the same stuff different names most of which is misused .
The difference is the shape of the graphite & it's distribution
like all iron carbon alloys it comes in a lot of different chemistries for different applications
Black heart & white heart are the most common terms used.

Work hardening is another term that has been so misused that it has become meaning less
When you cold from a metal the energy you put into the metal is stored within it as strain energy between the layers of atoms in each crystal
The more energy you put in the more strain gets stored in the lattice so the harder it becomes to move a type of irregularity called line disslocations .
Malleable irons are specifically designed not to work harden because the ferrite can shear at the grain boundries to relieve the stresses in the lattice so the linedisslocations are fairly free to travel through the crystals
If you do this very quickly a phenomenon called "high energy rate deformation" happens where the stressed crystals shear to become new smaller crystals
So now in your crankshaft you have chill crystals around the edges then directional growth crystals in the middle and a chunk of recryatalized crystals with a specifi orientation and specific slip planes all lined up in roughly the same plane which is the crank is a toroidal plain
Then you bend it back very slowly so some crystals deform a lot and other deform a little
Thus there is now a complex mix of planes of weakness through the section which previously was uniformly random .
Thus a much lower force than would normally be needed to cause the crankshaft to fail applied to the specific direction of the weak plain will cause a fracture .
So while the actual eleastic / plastic deformation transition point gets shifted higher the ultimate strength lowers and the slope of the stress strain curve above the transition point becomes virtually flat so you now end up with an instantaneous catastrophic failure .
The grains you stressed bending it back, are not the same ones that bent in the first place and these become stiffer due to the strain you introduced in the straitening.
The stiffer lattice resists the movement of line disslocations so lattice strain relief comes in the form of hydrogen atom diffusion.
And yes there are hundreds of millions of hydrogen atoms dissolved in nearly every metal that has not been degassed and cast in a vacuum.
The hydrogen atoms are very small so move easily and very quickly through the lattice till they bump into each other which surprise surprise will be along a slip plane and in particular a slip plane under stress.
The 2 hydrogen atoms then for a hydrogen molecule which further stresses the lattice which encourages more hydrogen atoms to migrate to the same place
The hydrogen atoms no longer are bonded to the lattice because the H-H bond is much stronger than the H-Fe or H-C bond so now you have an ever increasing void in the crankshaft commonly called an internal stress crack and this will continue to grow till the lack of crossectional area becomes so great the crank either bends again or breaks .
In the boxes of stuff you never throw out is a 10 page explanation of this with diagrams regularly presented as evidence to judges & juries and an even longer one with calculations that got used when the correct verdict got appealed against on the grounds of conflicting technical evidence .

Sheesh, where's the aspirin?

Walt Conner

for a bent shaft try http://bentcarrot.com

Confession: I have (partially) straightened a couple of lawn mower cranks in my time. I have been able, using pipes and hammers, to take enough of the bend out to restore usability to the mower. No doubt it is not a good practice.

Question: Is there a history of personal injury due to failure of "straightened" rotary mower cranks?

amuller said:

Confession: I have (partially) straightened a couple of lawn mower cranks in my time. I have been able, using pipes and hammers, to take enough of the bend out to restore usability to the mower. No doubt it is not a good practice.

Question: Is there a history of personal injury due to failure of "straightened" rotary mower cranks?

Click to expand...

Don't know of any personal injuries, but have seen several crankshafts on pushmowers sheared off even with the oil seal.

amuller said:

Confession: I have (partially) straightened a couple of lawn mower cranks in my time. I have been able, using pipes and hammers, to take enough of the bend out to restore usability to the mower. No doubt it is not a good practice.

Question: Is there a history of personal injury due to failure of "straightened" rotary mower cranks?

Click to expand...

I have never heard of such an injury. Straightened shaft or not. I have seen blades brake. It Never resulted in an injury. I've Never seen a shaft completely sever.

I have never seen a lot of things but I do know they happen

I have seen spindle shafts sheared off but never a push mower crank.